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DDC:
550 - Earth sciences
Abstract:
<P>The speciation of boron in H<FONT SIZE="-1">2</FONT>O+H<FONT SIZE="-1">3</FONT>BO<FONT SIZE="-1">3</FONT>±NaCl and H<FONT SIZE="-1">2</FONT>O+Na<FONT SIZE="-1">2</FONT>B<FONT SIZE="-1">4</FONT>O<FONT SIZE="-1">7</FONT> fluids was studied in situ at temperatures between 22 and 600°C and pressures from 0.1 MPa to about 2 GPa using Raman spectroscopy and a hydrothermal diamond anvil cell. Additionally, we determined the frequency shifts of the 877 cm-1 Raman line of [B(OH)<FONT SIZE="-1">3</FONT>]0 in aqueous fluids with temperature (<FONT FACE="Math1">d</FONT><FONT FACE="Math5">v</FONT><FONT SIZE="-1">877</FONT>/<FONT FACE="Math1">dT</FONT>)<FONT SIZE="-1">P=0.1 MPa</FONT> = -0.02532 cm-1K-1 and pressure (<FONT FACE="Math1">d</FONT><FONT FACE="Math5">v</FONT><FONT SIZE="-1">877</FONT>/<FONT FACE="Math1">d</FONT>P)<FONT SIZE="-1">T=22°C</FONT> = 4.06 cm-1GPa-1. The observed species in acidic fluids were [B(OH)<FONT SIZE="-1">3</FONT>]0 and smaller amounts of a four-coordinated boron species which may be attributed to dissolved metaboric acid HBO<FONT SIZE="-1">2</FONT>(aq). The ratio of this B[4]-O species to [B(OH)<FONT SIZE="-1">3</FONT>]0 increases with temperature and decreases slightly with addition of NaCl. In alkaline solutions, polyboric ions depolymerize rapidly with temperature. Thus, [B(OH)<FONT SIZE="-1">3</FONT>]0 and [B(OH)<FONT SIZE="-1">4</FONT>]- were the only remaining detectable species at 500 and 600°C. The Raman spectra showed an increase of [B(OH)<FONT SIZE="-1">3</FONT>]0 relative to [B(OH)<FONT SIZE="-1">4</FONT>]- with temperature and an increase of [B(OH)<FONT SIZE="-1">4</FONT>]- relative to [B(OH)<FONT SIZE="-1">3</FONT>]0 with pressure. <BR> The general trend in the boron speciation is a higher stability of simpler complexes with temperature. The experimental observations strongly indicate that planar three-coordinated [B(OH)<FONT SIZE="-1">3</FONT>]0 is the predominant boron species in the aqueous phase over a wide range of P-T-pH conditions. This supports the validity of previous assumptions on boron coordination in crustal and mantle wedge fluids.<BR> </P>